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My PC running on windows 10 was recently infected with a virus/trojan. I inserted a usb to it at some point , and connected it to my MacBook for clean formatting (I was using my MacBook to wipe it in hopes of getting rid of the virus).

I did not transfer any files from the usb to my MacBook, I have transferred a pdf file from my MacBook to the USB.

Under these circumstances, can my MacBook be infected with a virus this way?

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As I understand your question:

You used a known thumb drive i.e. not an unknown device that looks like a thumb drive.

You may have infected a file on the thumb drive from an infected Windows Machine.

You plugged the thumb drive into your Mac and formatted the thumb drive.

You wrote a file to the freshly formatted drive from your Mac.

If I have interpreted correctly, then there is no mechanism I've ever heard of that could have infected your Mac.

Even with a more cavalier approach, cross platform malware is rare. Relax, you're good!

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  • So, two clarifications:1. I did not transfer any files from the infected windows 10 PC system to the usb drive, the only interactions before I connect the usb drive to my mac is that I have connected this usb drive to my infected file and read the "list disk" option in command prompt. Before that the only files on my usb is the burn image of a windows 10 installation meida (it was a windows 10 installation usb). 2. 1. I DID wrote a file from my macbook onto the usb drive BEFORE I clean format it, However I didn't transfer any file FROM my usbdrive to my macbook. Does your answer still stand? – Ecotistician Apr 23 '20 at 11:35
  • Also, thank you so much for your response! – Ecotistician Apr 23 '20 at 11:37
  • @Ecotistician - There is a theoretical possibility but I've never heard of an actual exploit. You're good. – user10216038 Apr 23 '20 at 17:21
  • I see, does it make much difference (even at the theoretical level) between: 1. Having simply connected the usb drive to my macbook and formatted it 2. Having written on that usb drive before the formatting (aka file transferred FROM macbook to that potentially infected usb)? in terms of security threat (is 2. "riskier" than 1.? – Ecotistician Apr 28 '20 at 19:32
  • @Ecotistician - No not really. Macs (Windows too) will automatically mount and read a thumb drive looking for a hidden ID and write one if not found. I've never heard of any kind of exploit of that ID Check/Write function, but it's a theoretical mechanism. – user10216038 Apr 28 '20 at 22:33
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It would depend on the nature of the malware and the device, and what vulnerability it was targeting.

Without knowing any details, I would recommend you not trust the USB Thumb Drive.

FWIW, I would not recommend you trust any USB Thumb Drive you did not buy yourself or obtain from a trusted source (you are trusting them to have obtained it from a trusted source, ad infinitum).

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  • Ah shoot....I have already connected the usb drive to it (because at the time I thought it should be safe), I securely formatted the drive with my MacBook, scanned my MacBook with malwarebytes and avira and nothing turn up, what can i do now to first secure my macbook? As I'm trying to secure first my MacBook and then my PC. – Ecotistician Apr 21 '20 at 22:41
  • @Ecotistician As I said, it is hard to say without knowing what the malware was built to do. If the malware was only designed to work on Windows, then it may not have worked on your MacBook, but without knowing details I cannot say that with certainty. You did the right thing by scanning your MacBook for viruses. If you are still worried you could back up your data (photos, documents, etc.) and reinstall the operating system or contact Apple Support for help. – iraleigh Apr 21 '20 at 23:00
  • See @user10216038's answer. I may have misunderstood your question. If the USB drive was yours to begin with, I'll delete my answer as it is not relevant in that case. – iraleigh Apr 21 '20 at 23:58
  • It is mine, and it was clean before I suspect it may have became infected after connecting to a potentially infected windows 10 syste. – Ecotistician Apr 23 '20 at 11:31
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It really depends on how the malware was written and what triggers it to execute. We can't be sure if the malware has the ability to infect the peripherals and external storage. If it does we can't be sure if the malware also runs on a Unix environment which is drastically different from Windows. But always it's better to be safe than sorry. Run a scan on your Mac if you can, and pay special attention to system abnormalities.

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  • Well, you've expanded it, so that's all I was asking for – schroeder♦ Apr 21 '20 at 23:23

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